Co-liquefaction of sewage sludge and oil-tea-cake in supercritical methanol: yield of bio-oil, immobilization and risk assessment of heavy metals

In this study, attention was concentrated on the yields of bio-oil and toxicities of heavy metals (HMs) in liquefaction residues (LRs). Liquefaction of sewage sludge (SS) or oil-tea-cake (OTC) or mixtures of SS and OTC were carried out under the condition of supercritical methanol (SCM). Results showed that the addition of OTC extraordinarily increased the yields of oil from 37.9% (SS) to 86.2% (SS+OTC). Furthermore, with the liquefaction of SS and OTC mixture, the bioavailable fraction (F1+F2) of Cd and Cu (F1+F2) was decreased from 2.47 to 1.64mg/kg and from 98.84 to 67.48mg/kg, respectively. However, the bioavailable fraction of Zn (F1+F2) increased from 122.03 to 204.69mg/kg with the liquefaction of SS. The bioavailable fraction (F1+F2) of Pb in LRs was 0%, which did not express any changes during the liquefaction process. Risk assessments of geo-accumulation index (I-geo), risk assessment code (RAC) and modified potential ecological risk index (MRI) were applied to evaluate the bioavailabilities, the potential ecological risks and the pollution levels of HMs. The results show that the OTC in SS can decrease the risk of HMs in LRs. Cd attracted many concerns for the highest risk to the environment among all of the HMs. Here, the good results obtained means that SCM liquefaction of mixture of SS and OTC could be a preferable method for SS treatment.